1. Field of the Invention
The invention relates to a nonvolatile semiconductor memory device and a manufacturing method thereof, particularly, a one time programmable (OTP) memory and a manufacturing method thereof.
2. Description of the Related Art
An electronic tag for contactless identification of an object has been broadly used in recent years (e.g. physical distribution management, room access control, account management, and so on by a RFID tag). The RFID (Radio Frequency Identification) identifies and manages people or objects by a micro electronic tag, and is receiving much attention as a basic technology for promoting information technology and automation in a society as well as promoting a product identification and management technology replacing a bar code.
The electronic tag is attached to each of a large number of objects to be identified, and stores digital data about the attached object in its built-in memory. Generally, the digital data are contactlessly read and written with a reader/writer on the user side.
A mask ROM is an example of a memory storing digital data. The digital data in this mask ROM is fixedly stored when the mask ROM is completed. Therefore, after an electronic tag set with this mask ROM is marketed as a product, the user of the electronic tag can not write arbitrary digital data in the mask ROM.
Therefore, a nonvolatile memory such as EPROM or EEPROM in which the required digital data can be electrically written on the user side is used as a memory set in the described electronic tag.
However, the conventional electronic tag using EPROM or EEPROM as a memory has large die size and a complex manufacturing process due to the structure of EPROM or EEPROM, thereby increasing a manufacturing cost. This causes a problem of increasing a unit cost of the memory and the electronic tag set with this memory.
Therefore, it is preferable to use a one time programmable memory (referred to as an OTP memory) as a memory for the electronic tag or the like for minimizing the die size and the manufacturing cost. The OTP memory is written with data only once at the first time and then used without deletion of the data or addition of a program, as reflected in its name. Although the OTP memory is not often used as a product by itself since the stored data can not be changed, there are increasing demands for the OTP memory as a device for performing an auxiliary function in a semiconductor device as well as in the electronic tag.
A general OTP memory cell has a layered structure of a floating gate, a dielectric film, and a control gate formed in this order, as disclosed in Japanese Patent Application Publication Nos. 2004-193606 and 2004-356631. The relevant technology is also described in Japanese Patent Application Publication No. 2003-114247.
Usually, a semiconductor device has a few OTP memory cells therein, and the area of the OTP memory cells relative to the total area of a semiconductor product is not large. However, the general OTP memory cell has the layered structure as described above for obtaining nonvolatile characteristics. This layered structure is referred to as a PIP structure as a matter of convenience since a polysilicon layer, an insulation layer, and a polysilicon layer are generally formed in this order therein.
This layered structure (PIP structure) is formed by a complex manufacturing process, and increases a manufacturing cost of an electronic tag or semiconductor product with the OTP memory cell. Furthermore, the OTP memory cell is formed on a semiconductor substrate together with a high voltage cell transistor, which is relatively thick, to form an integrated circuit (since high voltage is applied in a writing operation), thereby increasing the die area.
Therefore, it is necessary to provide an OTP memory cell with high quality and a low cost in order to practically promote the semiconductor product or electronic tag with the OTP memory cell.